362 research outputs found
Magnetized Ekman Layer and Stewartson Layer in a Magnetized Taylor-Couette Flow
In this paper we present axisymmetric nonlinear simulations of magnetized
Ekman and Stewartson layers in a magnetized Taylor-Couette flow with a
centrifugally stable angular-momemtum profile and with a magnetic Reynolds
number below the threshold of magnetorotational instability. The magnetic field
is found to inhibit the Ekman suction. The width of the Ekman layer is reduced
with increased magnetic field normal to the end plate. A uniformly-rotating
region forms near the outer cylinder. A strong magnetic field leads to a steady
Stewartson layer emanating from the junction between differentially rotating
rings at the endcaps. The Stewartson layer becomes thinner with larger Reynolds
number and penetrates deeper into the bulk flow with stronger magnetic field
and larger Reynolds number. However, at Reynolds number larger than a critical
value , axisymmetric, and perhaps also nonaxisymmetric, instabilities
occur and result in a less prominent Stewartson layer that extends less far
from the boundary.Comment: 24 pages, 12 figures, accepted by PRE, revision according to referee
Three dimensional simulation of the magnetic stress in a neutron star crust
We present the first fully self-consistent three dimensional model of a neutron star’s magnetic field, generated by electric currents in the star’s crust via the Hall effect. We find that the global-scale field converges to a dipolar Hall-attractor state, as seen in recent axisymmetric models, but that small-scale features in the magnetic field survive even on much longer time scales. These small-scale features propagate toward the dipole equator, where the crustal electric currents organize themselves into a strong equatorial jet. By calculating the distribution of magnetic stresses in the crust, we predict that neutron stars with fields stronger than 1014 G can still be subject to starquakes more than 105 yr after their formation
Dissipative Taylor-Couette flows under the influence of helical magnetic fields
The linear stability of MHD Taylor-Couette flows in axially unbounded
cylinders is considered, for magnetic Prandtl number unity. Magnetic fields
varying from purely axial to purely azimuthal are imposed, with a general
helical field parameterized by \beta=B_\phi/B_z. We map out the transition from
the standard MRI for \beta=0 to the nonaxisymmetric Azimuthal MagnetoRotational
Instability (AMRI) for \beta\to \infty. For finite \beta, positive and negative
wave numbers m, corresponding to right and left spirals, are no longer
identical. The transition from \beta=0 to \beta\to\infty includes all the
possible forms of MRI with axisymmetric and nonaxisymmetric modes. For the
nonaxisymmetric modes, the most unstable mode spirals in the opposite direction
to the background field. The standard (\beta=0) MRI is axisymmetric for weak
fields (including the instability with the lowest Reynolds number) but is
nonaxisymmetric for stronger fields. If the azimuthal field is due in part to
an axial current flowing through the fluid itself (and not just along the
central axis), then it is also unstable to the nonaxisymmetric Tayler
instability, which is most effective without rotation. For large \beta this
instability has wavenumber m=1, whereas for \beta\simeq 1 m=2 is most unstable.
The most unstable mode spirals in the same direction as the background field.Comment: 9 pages, 11 figure
The first twenty exercise training program and fire academy recruits’ fitness and health
Master of ScienceDepartment of KinesiologyKatie M. HeinrichFirefighting is an inherently dangerous occupation with high rates of injuries and fatalities, with the majority of line of duty fatalities due to cardiovascular events. Additionally, firefighters struggle with poor health and low levels of fitness, including very high (>80%) rates of overweight and obesity likely related to the culture of the fire service. Limited resources exist for fire departments that are sensitive to the culture and work requirements of these “tactical athletes”. Though there has been increasing interest in circuit-type high intensity exercise training programs, key research data are lacking for the firefighter population and few studies have focused on training firefighter recruits. PURPOSE: The purpose of this pilot investigation was to examine a novel physical training program on fire academy recruits’ health, fitness, and performance, in addition to examining the programs’ acceptability, feasibility, and future efficacy. METHODS: Thirteen participants were recruited from an entry level fire academy and were randomly assigned to either the control (CG, n=6) or intervention exercise group (TF20, n=7). Due to attrition within the first two weeks of the study, 10 male fire recruits (23±3 years) completed the study (CG, n=3, TF20, n=7). The CG were asked to continue their current exercise habits. TF20 were provided an online-based training program (The First 20) that included periodized workouts, nutritional information, and mental readiness education. All participants completed baseline and post-intervention assessments and 10-weeks of exercise training. Health assessments included resting and post-exercise heart rate and blood pressure and estimated VO₂[subscript]max. Anthropometric measures included height, weight, % body fat, % lean mass, and BMI. Performance was measured using the Candidate Physical Ability Test (CPAT). Psychosocial measures were assessed by a short questionnaire. A feasibility analysis was also completed for those in TF20 group. Due to the small sample size and group differences at baseline, descriptive statistics were calculated and each participant was reviewed as an individual case study. The Wilcoxon Signed Rank Test was used to test for significance among TF20 group. RESULTS: This pilot investigation provided effect sizes and parameter estimates necessary for the design of a larger randomized controlled trial. Even with a small sample size, TF20 group showed improvement on numerous outcome measures including CPAT performance (40% passing at baseline to 86% passing post-intervention). Of five TF20 participants completing the CPAT at baseline and post-intervention, four improved their passing time. TF20 participants significantly increased estimated VO₂[subscript]max (p=0.028) and significantly improved body composition (decreased fat mass (kg) and % fat mass, p=0.028). TF20 participants also significantly improved grip strength (p=0.018). The CG saw no statistically significant differences from baseline to post-intervention. TF20 group completed approximately 75% of the assigned workouts. Participants reported enjoying the workouts and stated a program like this should be offered for fire academy recruits in the future.
CONCLUSION: While TF20 participants showed significant fitness gains, the small sample size limited comparisons to the control group or other covariates. TF20 program was well-received although there may be a better way to implement the intervention to increase participation. Participants mentioned they would like group workouts led by a certified strength and conditioning coach/peer fitness trainer as opposed to self-guided workouts. This investigation provides promising results for the efficacy of high-intensity training programs in firefighter recruits. Additionally, this study provides alternative guidance for exercise prescription designed specifically for the firefighter population
Wave propagation in semiconvective regions of giant planets
Recent observations of Jupiter and Saturn suggest that heavy elements may be diluted in the gaseous envelope, providing a compositional gradient that could stabilise ordinary convection and produce a stably-stratified layer near the core of these planets. This region could consist of semi-convective layers with a staircase-like density profile, which have multiple convective zones separated by thin stably-stratified interfaces, as a result of double-diffusive convection. These layers could have important effects on wave propagation and tidal dissipation that have not been fully explored. We analyse the effects of these layers on the propagation and transmission of internal waves within giant planets, extending prior work in a local Cartesian model. We adopt a simplified global Boussinesq planetary model in which we explore the internal waves in a non-rotating spherical body. We begin by studying the free modes of a region containing semi-convective layers. We then analyse the transmission of internal waves through such a region. The free modes depend strongly on the staircase properties, and consist of modes with both internal and interfacial gravity wave-like behaviour. We determine the frequency shifts of these waves as a function of the number of steps to explore their potential to probe planetary internal structures. We also find that wave transmission is strongly affected by the presence of a staircase. Very large-wavelength waves are transmitted efficiently, but small-scale waves are only transmitted if they are resonant with one of the free modes. The effective size of the core is therefore larger for non-resonant modes
Rapidly rotating plane layer convection with zonal flow
The onset of convection in a rapidly rotating layer in which a thermal wind
is present is studied. Diffusive effects are included. The main motivation is
from convection in planetary interiors, where thermal winds are expected due to
temperature variations on the core-mantle boundary. The system admits both
convective instability and baroclinic instability. We find a smooth transition
between the two types of modes, and investigate where the transition region
between the two types of instability occurs in parameter space. The thermal
wind helps to destabilise the convective modes. Baroclinic instability can
occur when the applied vertical temperature gradient is stable, and the
critical Rayleigh number is then negative. Long wavelength modes are the first
to become unstable. Asymptotic analysis is possible for the transition region
and also for long wavelength instabilities, and the results agree well with our
numerical solutions. We also investigate how the instabilities in this system
relate to the classical baroclinic instability in the Eady problem. We conclude
by noting that baroclinic instabilities in the Earth's core arising from
heterogeneity in the lower mantle could possibly drive a dynamo even if the
Earth's core were stably stratified and so not convecting.Comment: 20 pages, 7 figure
The benefits of using a walking interface to navigate virtual environments
Navigation is the most common interactive task performed in three-dimensional virtual environments (VEs), but it is also a task that users often find difficult. We investigated how body-based information about the translational and rotational components of movement helped participants to perform a navigational search task (finding targets hidden inside boxes in a room-sized space). When participants physically walked around the VE while viewing it on a head-mounted display (HMD), they then performed 90% of trials perfectly, comparable to participants who had performed an equivalent task in the real world during a previous study. By contrast, participants performed less than 50% of trials perfectly if they used a tethered HMD (move by physically turning but pressing a button to translate) or a desktop display (no body-based information). This is the most complex navigational task in which a real-world level of performance has been achieved in a VE. Behavioral data indicates that both translational and rotational body-based information are required to accurately update one's position during navigation, and participants who walked tended to avoid obstacles, even though collision detection was not implemented and feedback not provided. A walking interface would bring immediate benefits to a number of VE applications
Density-shear instability in electron magneto-hydrodynamics
We discuss a novel instability in inertia-less electron magneto-hydrodynamics (EMHD), which arises from a combination of electron velocity shear and electron density gradients. The unstable modes have a lengthscale longer than the transverse density scale, and a growth-rate of the order of the inverse Hall timescale. We suggest that this density-shear instability may be of importance in magnetic reconnection regions on scales smaller than the ion skin depth, and in neutron star crusts. We demonstrate that the so-called Hall drift instability, previously argued to be relevant in neutron star crusts, is a resistive tearing instability rather than an instability of the Hall term itself. We argue that the density-shear instability is of greater significance in neutron stars than the tearing instability, because it generally has a faster growth-rate and is less sensitive to geometry and boundary conditions. We prove that, for uniform electron density, EMHD is "at least as stable" as regular, incompressible MHD, in the sense that any field configuration that is stable in MHD is also stable in EMHD. We present a connection between the density-shear instability in EMHD and the magneto-buoyancy instability in anelastic MHD
New type of magneto-rotational instability in cylindrical Taylor-Couette flow
We study the stability of cylindrical Taylor-Couette flow in the presence of
combined axial and azimuthal magnetic fields, and show that adding an azimuthal
field profoundly alters the previous results for purely axial fields. For small
magnetic Prandtl numbers Pm, the critical Reynolds number Re_c for the onset of
the magneto-rotational instability becomes independent of Pm, whereas for
purely axial fields it scales as Pm^{-1}. For typical liquid metals, Re_c is
then reduced by several orders of magnitude, enough that this new design should
succeed in realizing this instability in the laboratory
Instabilities in magnetized spherical Couette flow
We report 3D numerical simulations of the flow of an electrically conducting
fluid in a spherical shell when a magnetic field is applied. Different
spherical Couette configurations are investigated, by varying the rotation
ratio between the inner and the outer sphere, the geometry of the imposed
field, and the magnetic boundary conditions on the inner sphere. Either a
Stewartson layer or a Shercliff layer, accompanied by a radial jet, can be
generated depending on the rotation speeds and the magnetic field strength, and
various non-axisymmetric destabilizations of the flow are observed. We show
that instabilities arising from the presence of boundaries present striking
similarities with the magnetorotational instability (MRI). To this end, we
compare our numerical results to experimental observations of the Maryland
experiment, who claimed to observe MRI in a similar setup.Comment: 11 pages, 12 figure
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